TBLISI, Georgia (CNN) -- Georgia's president said Friday that his country is under attack by Russian tanks and warplanes, and he accused Russia of targeting civilians as tensions over the breakaway Georgian region of South Ossetia appeared to boil over into full-blown conflict.

"All day today, they've been bombing Georgia from numerous warplanes and specifically targeting (the) civilian population, and we have scores of wounded and dead among (the) civilian population all around the country," President Mikhail Saakashvili told CNN in an exclusive interview.

"This is the worst nightmare one can encounter," he said.

Asked whether Georgia and Russia were now at war, he said, "My country is in self-defense against Russian aggression. Russian troops invaded Georgia."

About 150 Russian armored vehicles have entered South Ossetia, Saakashvili said, and Georgian forces had shot down two Russian aircraft.

Russia's Defense Ministry said it sent "reinforcements" to South Ossetia to help the Russian peacekeepers already stationed there. Watch the Russian tanks moving into the area »

The events followed an emergency session of the U.N. Security Council to discuss a dramatic escalation of violence in Georgia and South Ossetia. The session ended Friday morning without a statement about the fighting.

The Organization for Security and Cooperation in Europe (OSCE) said it was sending an envoy to the region immediately.

NATO Secretary-General Jaap de Hoop Scheffer issued a statement Friday saying he was seriously concerned about the recent events in the region, and he called on all sides to end armed clashes and begin direct talks. Watch more about NATO's attempts to help Georgia »

Carmen Romero, a NATO spokeswoman speaking to CNN from Brussels, reiterated Scheffer's statement. She said NATO was in regular contact with Georgia's president and was talking to the Russian side.

Britain and the United States also urged all sides to bring an immediate end to the violence.

"The U.S. has been in discussions for many months with all parties to find a peaceful resolution," said White House spokesman Gordon Johndroe. "We urge all sides to refrain from violence and to begin direct talks."

Earlier Friday, Russian military aircraft dropped two bombs on Georgian territory, a Georgian official said, causing no casualties.

In a letter addressed to his "fellow citizens" Friday, Saakashvili said he had mobilized tens of thousands of reserve officers and that the mobilization continued.

"We must unite," Saakashvili wrote. "All of us, hundreds of thousands of Georgians here and abroad, should come together, unite, and fight to save Georgia. We are a freedom-loving people, and if our nation is united, no aggressor will be able to harm it."

Georgia declared a unilateral three-hour ceasefire at 3 p.m. to enable civilians to escape from the conflict zone, which so far was focused inside South Ossetia but included aerial targets inside Georgia, Saakashvili said.

"Clearly they don't really have boundaries in their activities," said Georgian Foreign Minister Eka Tkeshelashvili, in an interview with CNN. She said Russian aircraft had bombed "several villages" in Georgia outside of the South Ossetian territory.

Tkeshelashvili said Georgian authorities are still collecting information on casualties.

Georgia was appealing to the world for diplomatic intervention, she said, stressing that Georgia was not asking for military assistance.

Violence has been mounting in the region in recent days, with sporadic clashes between Georgian forces and South Ossetian separatists. South Ossetia declared its independence from Georgia in the early 1990s, but its independence is not internationally recognized.

Georgian troops launched new attacks in South Ossetia late Thursday after a top government official said a unilateral cease-fire offer was met with separatist artillery fire.

Alexander Lomaia, the secretary of Georgia's National Security Council, said Georgian troops were responding proportionately to separatist mortar and artillery attacks on two villages -- attacks he said followed the cease-fire and call for negotiations by Saakashvili.

Russia said a Georgian attack on a military barracks left a number of Russian peacekeepers dead.

"It's all very sad and alarming," Russian Prime Minister Vladimir Putin said earlier in the day. "And, of course, there will be a response."

Putin was at a meeting with U.S. President Bush in Beijing, where they attended a luncheon for world leaders hosted by the Chinese president ahead of the Olympics, which begin Friday.

"There are lots of volunteers being gathered in the region, and it's very hard to withhold them from taking part. A real war is going on," Putin said, according to his spokesman, Dmitry Peskov.

Russian peacekeepers are in South Ossetia under a 1992 agreement by Russian, Georgian, and South Ossetian authorities to maintain what has been a fragile peace. The mixed peacekeeping force also includes Georgian and South Ossetian troops.

"The Georgian leadership has launched a dirty adventure," said a statement from Russia's Defense Ministry on Friday. "We will not leave our peacekeepers and Russian citizens unprotected."

Saakashvili said the Russian invasion of South Ossetia was pre-planned.

"These troops that are in Georgia now -- they didn't come unexpectedly," the president told CNN. "They had been amassing at the border for the last few months. They claimed they were staging exercises there and as soon as a suitable pretext was found, they moved in."

Georgia, located on the Black Sea coast between Russia and Turkey, has been split by Russian-backed separatist movements in South Ossetia and another region, Abkhazia.